The Room of Requirement could, in fact, provide further holodeck features as needed. It never again manifested anything that felt quite as prophetic as the fight in the Malfoy dining room (and Harry started to wonder if the other fights had been somehow relevant, rather than just because he'd had kree on his mind), but it did a good job of providing Death Eaters and marauders to fight.

They eventually learned that they'd gotten pretty lucky in the challenge fights, or maybe it was taking it a little easy on them: they started getting hit. And the Room was good at simulating damage. Wounds that would be painful but not incapacitating hurt just as much as they would have, glowing with a red fog to indicate where the wound was. If the harm should be enough to disable a limb, that arm or leg would be paralyzed with that same red fog pulsing along its length. Killing attacks would result in a full-body cocoon of the energy, freezing the victim on the floor until the simulation ended.

But, hey, it was still loads better than having to learn to deal with getting hit for real the first time, Harry figured. The girls didn't really see it that way, of course, and redoubled their efforts to let the boys tank the hits while they served as artillery.

They also quickly discovered that they could bring other people in without having to do another bout of challenges, as long as they entered with someone else. They'd sent Colin Creevey in ahead of them the first time, and he'd come running back out almost immediately rather than face the Nidhogg serpent the Room had thrown at him solo. It seemed he still had some trauma from his first year.

"We should probably invite more people than just the Gryffindors we like," Parvati opined, a couple of days before the Hogsmeade weekend at the end of Vintage Month (around October 20th, back on Earth).

"How many people already know about it?" Harry checked. He tried to keep his tone neutral, but with Parvati and Lavender involved, he'd realized it wouldn't stay a secret for long.

"Mostly just Gryffindor!" she retorted, picking up on the subtext. "...and a couple of Hufflepuffs who are cool. Cho, over in Ravenclaw. No Slytherins!"

"I told Theo," Seamus admitted, contradicting her assertion.

"We need to put a lid on it before the professors find out," suggested Hermione. Everyone looked at her like she'd been replaced by her goatee-wearing doppelganger. "What? They'd want us to have adult supervision and Dar-Benn would figure out how to make that her, defeating the whole purpose. I'd show it to Master Mordo, but no telling what he'd say about it, and I don't want to risk it."

"'This is an abhorrent and wasteful use of magic and I want no part of it! Who knows what kind of dimensions this calls upon, or how it weakens the walls of reality!?'" Parvati mimicked the dour sorcerer. She still was not his biggest fan. "And, yes, I know what 'abhorrent' means! I study."

"Why don't we make a club? And get people to sign a pledge that they won't tell anyone outside of the club?" Lavender suggested.

"I could make it a binding magical contract!" Hermione agreed, having been researching them since Dar-Benn tried to trick Harry with the blood pen. "Also, yeah, we should pitch it as a way to practice against marauders and Death Eaters. We can't be the only ones that need to know that."

That's how they found themselves staking out a seedy pub on the side of Hogsmeade that students didn't usually frequent. The place was opposite the main square and away from the train tracks, a little ways away from the main mass of town. Most kids mistook it for a farmhouse, with the goat pens in the back. But it was a cheaper drinking hole for locals, and, it turned out, didn't actually mind a bit of coin from students.

That was being sorely tested with the sheer number of them.

The main benefit of the establishment was enough distance from the rest of town that their lookouts would see a set of magenta robes coming from far enough away to cease the intended business and pretend that it was a normal social gathering. Parvati and Lavender had put out the word about what was happening to those they trusted, and the others had hit up a few people in the various other houses that they knew. Consequently, the place had filled up with over a third of the students that were at Hogsmeade that weekend.

Virtually all of Gryffindor had turned out, a dozen Ravenclaws, half a dozen Hufflepuffs, and Theodore Nott from Slytherin (he said he'd clue them in on which of the other snakes were cool, if this seemed worth his while).

"Okay, everyone!" Lavender called out to get their attention. "We realized that some of the rest of you might like to have extra practice for how to fight marauders and Death Eaters. We found the Room of Requirement, so we can do some excellent simulations. Harry's fought a bunch of bad guys already, and can teach us. A lot of other Gryffindors have practical experience, too."

"Is this just a dueling club, like a couple years ago?" Finch-Fletchley asked. He hadn't had the best experience there, or the evening after.

"No. Team battle practice," Dean answered. "We're mostly going to work on what it's like to fight against groups of warriors who might have dark wizards backing them up. How to use teamwork to protect each other."

"Do the rest of us get to show off our expertise, or is this just the Harry Potter show?" McLaggen asked.

"Only if you can prove yours works, Cormac," Ginny insisted.

"The news says you're basically full of it, Potter," one of the Hufflepuffs in the year behind them, Smith, tagged in. "Why should we learn from you? If it's just marauders, we can wait for Asgard to finish this up."

Harry shrugged, and explained, "There are already way more people here than I expected. If you don't want in, then that's one less person I have to train. It's not just marauders, though. They definitely have wizard support. If you don't want to believe they're the real Death Eaters from the last war, that's fine. They're still assholes that are trying to use force to take your stuff."

"I have a sign-up sheet here," Hermione cut the discussion short. "If you want in, you're agreeing to not tell anyone how to get to the Room of Requirement, including teachers. But especially Dar-Benn. We don't trust her, or want her to know what we can do."

"And if you don't like the instruction, you don't have to keep wasting your time," Harry tagged in. "All we ask is that you keep the Room a secret, even if you quit."

"First meeting will be next Sunday, the day after Dísablót, if it matters for your decision," Lavender remembered.

Ultimately, about forty students signed up, not counting the study group, so they had around a fifth of the entire student body. No doubt a lot of them were just there for Harry's fame, either to spend time with the celebrity or to prove to themselves that he was no big deal. But maybe they'd learn something.

First, they needed to make it through Halloween, where something bad often happened at the school.

Harry actually went to the feast that year. It wasn't his favorite day ever, but, oddly, all the weird things that had happened to him on Halloween over the last few years had softened the association it had with his parents' death. It was still a day where he waited for something to go wrong (even more than on his birthday), but he was anxious rather than sad. And that, strangely, counted for a lot.

Plus he was a teenaged boy with a functionally-bottomless stomach and had always been sad to miss one of the better food events of the year.

There was no troll attack. No petrified cat. No flaming goblet. Not even the weirdness of starting up a dating test with his friends. Things were basically just food with a spooky atmosphere. Without Hagrid around to grow giant pumpkins, even the jack-o'-lanterns were normal-sized. Some were even small.

That much normalcy was clearly too much for the students, who had all kind of gotten used to something exciting happening on Dísablót. There was a tension in the air, just waiting to earth itself. So when Fred and George quietly started putting the word out toward the end of the meal that they'd pay two galleons to anyone that would hit a Slytherin, preferably Malfoy, in the head with a treacle tart (five, if they'd hit Dar-Benn), the room was poised to erupt. One perfect slice of sticky pie onto Draco's well-gelled platinum coif from an enthusiastic second-year Gryffindor, and it was on. The room erupted into flung deserts.

The first few people to initiate the food fight are the ones that need to have bad risk-assessment: if nobody else joins in, they're getting the full brunt of punishment. But each additional flung plate of food reduces the overall burden. By the time everyone is food fighting, who are the teachers going to punish, really?

It didn't hurt that Ginny, eager to keep her brothers' money in the family, used the distraction of the first few volleys to levitate a tart over onto Dar-Benn. And then the rest of the teachers were so amused they were less inclined to put a sudden halt to the fun. By the time ten deserts had flown, there were no longer going to be repercussions. Watched from the outside, the ramp into full sugary war would have been a fragile crescendo.

Harry and Dean took turns with one shielding while the other flung handfuls of cake at no particular target. Ron got the first-year Gryffindors organized and throwing pumpkin pudding at targets that were distracted in the neighboring tables. Luna produced a set of gravity-defying plums from somewhere and got them to hover over and drop on her roommates. Hermione, Neville, and Padma had enough sense for their prefect status to just hide under the tables but not try to stop anyone. Lavender and Parvati were sauntering over to various tables and pieing people directly in their faces. Seamus figured out how to make a flambé out of illusory fire, throw it, and make some Slytherin that was mean to Nott squeal in fear and dive out of the way.

Little Ned Leeds kind of stole the show, having jumped onto the Ravenclaw table and started hucking whole jack-o'-lanterns at kids that had been mean to him. It was a good time.

Eventually, even with the massive spread, there was no more food to throw, and the fight wound down. The teachers had seemingly all been able to shield themselves, remaining pristine. It was strange that they hadn't tried to shield Dar-Benn. They'd protected Madam Grubbly-Plank, who also couldn't shield herself. It was a mystery. "What an exciting end to our festivities," Dumbledore announced, five seconds after Colin Creevey had flung a whole cookie like a frisbee as the last piece of available ammunition. "Your heads and prefects will nominate four enterprising individuals from each house to stay behind and practice your cleaning charms, lest we leave a sticky mess for the staff, or encourage the castle's equally-enterprising ants. Once those have been chosen, the rest of you are free to return to your dormitories and hope that our hot water lasts."

It was weird how quickly McGonagall chose the entire Weasley clan in residence as Gryffindor's four who had to remain and clean.

A very satisfying shower later, Harry was in bed and dreaming. He hadn't had too many dreams of Morag in the previous weeks, but that night was one of them. His vision flickered over the slowly-emerging mountain island, revealed a Vanir woman in a green shawl sitting in the spot he'd originally seen Arthur Weasley, and then began to descend. The watery depths of the planet were as alien as the ocean of any other world. What differed was the franticness of the marine life. Creatures that were not quite fish seemed keenly aware of the lowering surface above their heads, trying to get enough to eat and then disappear into the heart of the planet that remained watery.

At a depth still great enough to endanger the unprotected and murky enough to make it virtually impossible to search for an Infinity Stone, Harry's vision roamed. Once, this entire place had been a city, and the Stone had been interred somewhere as part of its religion. Ruined buildings loomed out of the silt, impressive in that they'd survived multiple cycles of the waters rising and lowering. They were the lucky ones, the rest of the space barely providing evidence that a society had ever existed there.

How many more cycles would the holdouts last? When the Stone was taken, would fate let the remainder be torn apart by the waves?

The next day was time for the meeting of the new club for which they were still workshopping the name. Hermione had written "Defense Association" on the signup contract because nobody had really had a better idea, but it lacked some kind of pizazz. Maybe somebody would have a brainwave later. At breakfast, the study group managed a "pass it on" campaign to tell the various members who'd signed up (and who they should tell) to meet near one of the classrooms on the fifth floor in about half an hour.

Importantly, the spot they picked was away from any professors' offices and fairly close to a secret passage up to the seventh floor. With the twins (still a little worn out from all the cleaning of the previous night) watching the Map for adult interference, Harry met up with everyone to show them the passageway (ready to turn invisible at warning from his lookouts), and the rest of the study group took turns escorting people into the Room as they reached the seventh floor.

At least for the first meeting, Dar-Benn seemed to have no idea. The twins reported she'd stayed put in her office the whole time. It was also weird, they opined, the way the students in the Room weren't anywhere on the Map.

"Everybody!" Harry announced as he led the twins into the Room after theoretically everyone who was coming had gone in. As the assembly went quiet he said, "Thanks for coming. A lot of you have probably seen our crew doing various workouts on the grounds. Probably a lot of you would also benefit from cardio. Today, we're going to start off by seeing how you'd actually do in a fight. Try not to hit any other students with anything, okay? Otherwise, anything you can cast, goes. With me? Great! Room? DEATH EATERS."

It was absolute chaos. Simulated enemies in black cloaks and silver masks began smoke-teleporting into the midst of the students present. About half of the kids screamed and tried to find cover. Another quarter gamely tried to fight but didn't work together, and were sporting the room's red incapacitation smoke within ten seconds of the fight starting. The rest managed to actually do something productive, assisting the study group, who were pretty good at working together by that point.

The whole fight was over in about a minute. Some of the kids who'd hidden had been found and tagged by simulated bad guys. But being out didn't mean you couldn't watch. Harry led the study group and about six other kids that'd gotten through without serious injury to sweep the remainder into defeat, not afraid to let loose with some seriously painful spells.

When the last Death Eater evaporated back into red mist, and the kids that had been incapacitated got back to their feet, Harry started talking again. "In a game several of us like to play, combat is measured in rounds of six seconds. It's a dumb, arbitrary number. A lot of the time, you'll take a few rounds to try to get in position and not be noticed by your enemies. You can spend minutes just running in between fights. But when you actually get in a life-or-death battle, it's so fast. Look at that clock." He gestured to a large clock that had been placed on one wall and started ticking when he started the fight. "All of that took sixty-six seconds. Dozens of friends and enemies. Everyone was dead who was going to die that fast.

"I'm not here to tell you that you should have done something differently. I'm here to show you ways to work together and keep it together so at the end of that minute or whatever, you're one of the people still standing."

Even Zacharias Smith and Cormac McLaggen had to admit that Harry Potter had been damned effective. They mentally consoled themselves that he was in control and had stacked the deck so he could look effective, but they hadn't done anywhere near as well. And none of his friends had gone down either.

"Everybody with me?" Harry continued, seeing that he finally had real buy in. "Let's talk about the Holy Trinity: tanker, nuker, healer…"

The rest of the first meeting went well. They set up a schedule of a few more before the winter holidays, and largely expected everyone to return. Of course, the study group still planned to use the room more often than once every couple of weeks, but they felt good about so many other students getting more comfortable protecting themselves and their friends.

Before the next meeting, it was the Gryffindor versus Slytherin quidditch match. Harry was still alternate for seeker, and the house team was now 57% Weasley. With the twins dating Angelina and Alicia, that left Katie Bell as the only member that wasn't even Weasley-adjacent. Lavender did not enjoy the jokes about Ron dating her to fix that, nor did Dean like the jokes about Ginny dating her instead.

"She's going to stay on Vanaheim, dude," Harry reminded Dean, as he was complaining about it the morning before the match. "It's not like it's going to last past graduation anyway."

"Yeah, but I'm enjoying dating her now," his best friend argued. "She barely even still seems hung up on you."

"Good. I don't know how she'd do it between worlds, but Fleur would kill me if I even thought about dating Ginny."

"The long distance thing is still going okay?"

"I mean… it's still not technically even a thing," Harry admitted. "But the last time I took a day longer than usual to write back, she got a little mad. Or worried. Or worried and mad. So I figure it's kind of a thing, right?"

"Your not-girlfriend is on another planet and engaged to an elf prince, but Ginny staying on Vanaheim when we graduate is an impossible obstacle?"

"Touche."

The Gryffindor team was a well-oiled machine. They'd lost Oliver Wood, but Ron had been practicing to replace him for years. The rest of them had been on the team since Ginny had replaced Harry as seeker. Meanwhile, Slytherin had lost their original captain, Marcus Flint, and a couple of other upper-classmen. In true Slytherin style, the boys who'd graduated had kept the high-end brooms the Malfoys had bought for them, rather than leave them for their replacements, so the team didn't even have an overwhelming equipment advantage.

It seemed like Malfoy had bigger concerns out in the world than helping his son cheat at school sports.

Harry had mentioned to Dumbledore and Sirius that it was possible the Lestranges were hiding out at Malfoy manor (not really pointing out to the headmaster that the idea was founded on a scenario created by the Room of Requirement). Unfortunately, until the man himself was revealed (or at least strongly suspected) to be a Death Eater, there was no legal cause to search his holdings. They'd really just need to catch him in the act, as soon as possible.

Regardless of the inability to catch him harboring fugitives, at least they were able to embarrass his son on the field. Slytherin lost again, and lost badly.

Perhaps more interestingly, the day after the match, Hagrid was back to teaching husbandry classes. He looked like he'd taken some damage and been living rough for months, but he didn't elaborate on where he'd been. He did mention his time with Maxime—the other half-giant from Beauxbatons—a couple of times over the next week, so most of the school just wrote it off as him going on some long romantic getaway (his wounds inspiring them to not ask any more questions).

But the study group wasn't like most of the school.

"On some trip for Dumbledore. Probably on Jotunheim, if the frostbite he has on his left pinkie is any indication," Hermione figured.

"I'll ask Fleur if she's heard anything about Maxime," Harry agreed.

"There have been some jotuns in the marauder groups," Lavender offered, their finger on the pulse of Vanaheim news. "Maybe he was trying to get them to stay home."

"I wonder if it worked," Neville worried.

Before they could make any headway convincing the big man to accidentally reveal what he'd been up to and satisfy their curiosity, it was on to more DA meetings and end-of-term papers. There was a lot of progress in both. Weirdly, there was even synergy. Teens that normally wouldn't really have much to say to their peers in other dorms mentioned homework in lulls between combat scenarios. Harry had been deliberately grouping teams cross-house as much as possible, after all, to try to prevent obvious cliques from forming. For all his bluster, Zacharias Smith was an actual resource on several topics, as were several of the Ravenclaws.

The last of their tests and essays out of the way before winter break, they had time for one last DA meeting. It was getting a little harder to get everyone into the Room, as some of the professors had noticed how quiet the school was on those days, and Dar-Benn had started patrolling the hallways, likely trying to find them. They all hoped the adults would forget about it over the break. Harry re-ran the initial scenario, of Death Eaters teleporting in, and it went much better. Only 42 seconds to victory, and many fewer casualties among the students. Everyone was able to head to the break with a strong sense of accomplishment.

That night, Harry dreamed another vision of Morag. Dumbledore's watchers had needed to move from their original lookout spot further down the mountain, lest they just be staring down over a hundred yards to the water. Unfortunately, the new space wasn't as sheltered. And someone was taking advantage of that.

In Harry's sight, Arthur Weasley was once again on duty, a small fire at his feet keeping him warm as he sat in a camp chair clearly obtained from Earth. But the vision rolled back, and Harry realized he could see the fire from a ways down the "beach" that was the temporary waterline. And, suddenly, it was eclipsed by a truly massive form. A hulking, scaled humanoid with horns or crests on his face and chin was moving down the beach with surprising stealth for such a behemoth. He wore armor that made it clear he wasn't just some local wildlife, and he was heading straight for Mr. Weasley, dragging a high-tech scythe like he meant to use it.

Harry managed to wake himself up from the vision and started yelling at his roommates, "Everyone, up! Up!" he fumbled for his bag of holding and managed to grasp his hand mirror as the four other boys started to groggily wonder what he was yelling about. "Sirius Black!" he activated the mirror, hoping his godfather would pick up.

"Harry? Wha?" Neville was the first up.

"Go get McGonagall. Dumbledore if you can," he told them just as Sirius' equally-groggy face appeared on the mirror. Before the adult could also make questioning syllables, Harry explained, "Mr. Weasley is on lookout, right? There's some kind of giant lizard man coming after him. Right now! Can you get in touch with him? Can you get someone over there to help?"

Ron had picked up on his father's name even through his stupor, and was already sprinting out of the room to look for a teacher that might be able to help, Neville not far behind him. Sirius, equally instantly awake, said, "I'm on it. Tell Albus." Then he cut the connection, hopefully to make other calls.

And then the waiting game began.

After being up for an hour in the dark with no word, Harry started to worry that Sirius had run to help and had also been overcome by the enemy. But finally, Neville came back and told him, "Headmaster wants to talk to you. I also woke up Fred, George, and Ginny to get their stuff and go with us. We're supposed to take Ron's stuff, and get yours as well. Everything you were going to take home for the hols."

The walk across the castle was tense. Harry just told the other Weasleys, "I saw a vision of your dad about to get attacked and tried to send him help. I don't know what happened." Even the twins weren't able to joke about something like that.

Dumbledore had thrown on a star-spangled dressing gown and hadn't even put on a hat when he let them into his office. Ron bolted out of his chair to hug his siblings, not even able to marshal the general teen standoffishness around family. "He's going to be okay!"

"It was near thing," the headmaster elaborated. "If Harry had been a minute later, it might not have been. As it was, Sirius, Arthur, and a few of our other allies were seriously injured, and will be some time recovering. But they managed to hold the enemy off long enough to escape."

"Not beat him, sir?" Harry questioned.

"I believe Hagrid never described to you exactly from whom he got the dragon egg in your first year?" the old man asked.

"Some guy in the market?" Harry vaguely remembered. "No, wait… it was a dragon, so it kind of made sense that he got it from a… a lizard man!?"

"Of the same description as the one we just fought," he agreed, sadly. "I believe this to not just be any marauder, but a key operative for our true foe. Against even five wizards without access to the power of their wands… it was good that they walked away with injuries from which they will recover. And that Arthur wasn't taken away to be interrogated, or even worse."

"You saved Mr. Weasley," Neville realized, tearing up a little as he wished that someone with Harry's powers had been in time to save his own parents from a similar fate.

"He did," Dumbledore agreed. And suddenly Harry and Neville were both being pulled into the Weasley group hug.

While he would have preferred if Mr. Weasley, Sirius, and an unknown set of the headmaster's spies hadn't been hurt right before the holidays, or that they'd managed to defeat their brutish opponent, nevertheless, he'd take the win.

It would have been a pretty terrible Christmas if someone he knew got killed.